1. Field of the Invention
This invention relates to a fuel composition, particularly to a fuel composition comprising a heavy oil-based fuel including heavy oil, coal slurry or COM and an additive for such a fuel.
2. Description of the Prior Art
Heavy oil-based fuels such as heavy oil, coal slurry and COM contain considerable amounts of carbon residue and moreover their ignition speeds are low. Therefore, their combustion is unstable and evolves a lot of unburned carbon or soot. It is hence necessary for such a fuel to use excess air at a high level, leading to reduction of the combustion efficiency. With this disadvantage in view, it has been attempted to add an combustion promoter to the fuel so as to suppress the evolution of soot, etc. even when the fuel is burnt with a low level of excess air. As the combustion promoter, there have been proposed fine particles of hydroxides or oxides of divalent to tetravalent metals which have adsorbed naphthenic acid, etc. (Japanese Patent Laid-Open No. 152794/1986) and iron oxide particulates comprising not less than 80% of the particulates with sizes of 1 .mu.m and smaller (Japanese patent Laid-Open No. 106992/1987) by way of example.
However, these fuel additives are solid and therefore do not mix with the fuel so intimately. Moreover, the additives may precipitate if it takes a long period of time from their addition to the use of the fuel. Further, these additives are required in relatively large amounts to exhibit their effects, thereby rendering the addition operation complicated and giving adverse effects on the efficiency of dust collectors.
On the other hand, it has been disclosed that a basic and oil-soluble cerium soap with not more than 3 equivalents of acid to the cerium atom is useful as a combustion additive (Japanese Patent Laid/Open No. 12907/1978). However, there has been a strong desire to develope combustion additives with further improved combustion promotive effects.